Hey, pretty cool stuff!
Well it'd be coming along great if you were going for a highly detailed portrait, but looking at the other portraits that doesn't seem to be the intention. I would say the others make steps to simplify forms quit alot, and focus on texture for strategic detail.
I think all you really have to do is dial back the detail just a little, and when you start playing with the palette to bring out contrast and saturation, well I think it'll come up sweet.

Oh as for pale-green tinge, maybe something like this?:

Kind of, have an injection of redness (arrow in pic) towards the terminator (think I used that term correctly), and then drop into grey-greens. I think that's the best way to go for human-lizard thingo (not necessarily these exact colours, though).